What is Wettability?

The term “wettability” might be a novel term to you, but this is very much related to physics.

Wettability is the tendency of a fluid to adhere to or to spread on a solid surface. This tendency of the fluid is tested in the presence of various other immiscible fluids. In simpler words, it can be said that wettability is basically used to measure how much a fluid likes to stay in contact with a surface. In this article, we will discuss this in greater detail.

Degree of Wettability

The degree of wettability is measured by the balance of force between the adhesive and cohesive forces. The adhesive forces acting between a liquid and a solid cause a drop of liquid to spread across the surface. The cohesive forces acting within the liquid cause the liquid drop to avoid contact with the surface.

Explanation

Refer to the image shown below. “θ” is the contact angle. The contact angle is defined to be the angle of contact between the liquid-vapor interface and the solid-liquid interface.

Contact angle

If the tendency of a drop of fluid to spread out over a solid, flat surface increases, the angle of contact decreases. Therefore, it can be said that contact angle is inversely proportional to wettability.

A low contact angle (<90°) usually has the indication that the wetting of the surface is pretty favorable thereby, confirming the fact that the fluid will spread over a greater surface area.

A high contact angle (>90°) means that the wettability of the surface is unfavorable. Hence, it can be said that the wetting of the surface is NOT favorable. Thus, the fluid will minimize its contact with the surface and will finally form a compact liquid drop.

In case of water, a wettable surface is termed hydrophilic. A nonwettable surface, on the other hand, is termed hydrophobic.

Behavior of Liquids

The wettability of a surface is used to determine the behavior of liquid. You’ll notice that liquids have a tendency to stick to solids. However, the molecules present in a liquid are also attracted to one other. This results in the phenomenon of surface tension. It’s due to this surface tension; liquids form tiny droplets on the surface of a solid.

Point to be noted- Wettability varies strongly with surface roughness. Let the surface be a rock. The wettability of rock will vary greatly with grain shape, size as well as rounding.

We have come to the end of this article for now. But before signing off, we would like to point one thing out. Do you remember that your teapots drip?

Well, now you know the reason behind it. It's due to the factor of wettability. With that, we’ll finally sign off for the day. Hope you had a good read.

Sudipto Das

Sudipto writes technical and educational content periodically for wizert.com and backs it up with extensive research and relevant examples. He's an avid reader and a tech enthusiast at the same time with a little bit of “Arsenal Football Club” thrown in as well. He's got a B.Tech in Electronics and Instrumentation.
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